In an air-conditioning apparatus such as a multi-air-conditioning apparatus intended for multistory buildings, refrigerant is made to circulate between, for example, an outdoor unit that is a heat source unit provided outside the building and indoor units provided in rooms of the building. As the refrigerant transfers or receives heat, air is heated or cooled, whereby heating or cooling conditioned space. Refrigerants such as HFC (hydrofluorocarbon) refrigerant are frequently used. Further, apparatus using natural refrigerant such as carbon dioxide (CO2) have also been proposed.
In an air-conditioning apparatus called a chiller, a heat source unit provided outside a building generates cooling energy or heating energy; a heat exchanger provided in an outdoor unit heats or cools water, antifreeze, or the like; and the heated or cooled water, antifreeze, or the like is conveyed to indoor units such as fan coil units or panel heaters, whereby cooling or heating is performed (see Patent Literature 1, for example).
There is another apparatus called a heat recovery chiller in which four water pipes connect a heat source unit with each indoor unit, and cooled water or the like and heated water or the like are supplied simultaneously, whereby making cooling or heating arbitrarily selectable in each of the indoor units (see Patent Literature 2, for example).
There is yet another apparatus that provides heat exchangers for a primary refrigerant and a secondary refrigerant near respective indoor units, in which the secondary refrigerant is conveyed to the indoor units (see Patent Literature 3, for example).
There is still yet another apparatus that connects an outdoor unit and branch units with heat exchangers to each other with two pipes, in which a secondary refrigerant is conveyed to the indoor units (see Patent Literature 4, for example).